Improvements to MocoUtilities::createExternalLoadsTableForGait()

[nickbianco]

Fixes issues #3763, #3909

Brief summary of changes

Add the two following improvements to MocoUtilities::createExternalLoadsTableForGait():

1. When vertical force is zero, centers of pressure are set to zero, rather than be computed as NaN.
2. The utility now returns vertical torque instead of the sum of sphere torques. Now, the forces, torques, and COPs should all be consistent with each other.

Testing I've completed

Added a test to testMocoContact to check that the new representation of GRFs produces the same accelerations as the original contact force eleents.

Looking for feedback on...

CHANGELOG.md (choose one)

• updated.

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This change is 

[carmichaelong]

LGTM. Just some minor suggestions for documentation clarity. Feel free to merge after addressing those (if you do).

Reviewed 5 of 7 files at r1.
Reviewable status: 5 of 7 files reviewed, 2 unresolved discussions (waiting on @nickbianco)

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CHANGELOG.md line 27 at r1 (raw file):

  comprehensive overview).
- Made improvements to `MocoUtilities::createExternalLoadsTableForGait()`: center of pressure values are now set to zero, rather 
  than NaN, when vertical force is zero and the vertical torque is returned in the torque columns (rather than the sum of the 

these are two statements, right? i.e.:

1. center of pressure values ... is zero
2. the vertical torque ... GRF representation

if i'm interpreting right, could split into two sentences, or just add a comma, e.g. when vertical force is zero, and the vertical torque...

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OpenSim/Moco/Test/testMocoContact.cpp line 623 at r1 (raw file):

// This is a round-trip test: we create external loads from a trajectory
// using createExternalLoadsTableForGait(), then use the external loads to
// apply forces to a model, and ensure the accelerations of the model match

consider something like the following to provide an overview of the test at the top (the comment by the last for loop was helpful for this):
... apply forces to a model without contact, and ensure the accelerations match those of the original model.

[nickbianco]

Thanks @carmichaelong!